The invention relates to dual band antennas including but not limited to dual band feeds for reflector antennas. The invention also relates to a carrier casting for a dual band antenna.
Domestic satellite communication antennas are widely used to receive signals such as television broadcasts rather than to transmit as well as receive. However, demand for interactive services such as interactive television and use by small office/home office users has led to the requirement for domestic two-way satellite communication to be provided.
This is possible by using two antennas, one for an up-link or transmission signal and one for a down-link or reception signal. However, this increases the cost of the equipment needed by a subscriber and also increases installation, transport and maintenance costs. The space required for the antennas is also greater and this is a particular problem for domestic applications where space is at a premium.
The up-link and down-link signals are provided at different frequency bands in order that they are readily distinguishable and do not interfere. Antennas which provide two frequency bands are referred to as dual band antennas and a number of different types of dual band antennas are known. However, these suffer from a number of drawbacks when considering subscriber satellite communication systems.
For example, frequency selective surfaces can be used to provide dual bands as in earth station antennas. FIG. 1 is a schematic diagram showing use of a frequency selective surface 131. Signals from a transmitter 131 reflect from the frequency selective surface 133 and onto a reflector 130. However, signals received at a different frequency and reflected from reflector 130 towards the frequency selective surface pass through that surface 131 towards a receiver 132. That is, the frequency selective surface is arranged to reflect signals of a certain frequency range and transmit others. In this way dual band communication using only one main reflector 130 is possible. However, this type of system is difficult and expensive to install because four components, the transmitter 131, receiver 132, frequency selective surface 133 and reflector 130, must all be correctly aligned. This is difficult to achieve at low cost. Another problem is that cabling must be provided to the transmitter and receiver separately because these have different locations. This also increases installation costs.
Another approach has been to provide a dual band feed for a reflector antenna. For example, this type of system is described in U.S. Pat. No. 4,740,795, Seavey. Two coaxial waveguides are used for the respective two frequency bands and in order that the beamwidth of each beam is similar (and arranged to cover the reflector surface) these waveguides are of different diameter. In order to accommodate this arrangement the design is complex and expensive. In addition, dual band feed systems such as that described in Seavey are not suitable for monopulse alignment methods or for distributed power amplification.
Monopulse alignment methods enable an antenna to be accurately aligned with respect to a satellite and this is particularly important in subscriber satellite communication applications where there is typically little room for alignment error and where costs for an operator to align an antenna are high. Distributed power amplification is advantageous because high power transmit amplifiers are not readily available at millimetric frequencies. In dual band feed systems such as the Seavey system, distributed power amplification is not possible because there is only one transmit antenna element.
U.S. Pat. No. 4,141,012, Hockham et al. describes a dual band waveguide radiating element for an antenna. Using this element an array antenna which operates at two frequencies can be provided. The waveguide element is excited by probe structures entering the guide perpendicular to the plane of the array face. This has significant cost and size implications because the antenna is not a xe2x80x9cflat-platexe2x80x9d. Also, in terms of the number of elements being fed the approach described in U.S. Pat. No. 4,141,012 is inefficient.
A general rule in antenna design is that, in order to xe2x80x9cfocusxe2x80x9d the available energy to be transmitted into a narrow beam, a relatively large xe2x80x9caperturexe2x80x9d is necessary. The aperture may be provided by a broadside array, a longitudinal array, an actual radiating aperture such as a horn, or by a reflector antenna which, in a receive mode, receives a collimated beam of energy and focuses the energy into a converging beam directed toward a feed antenna, or which, in transmit mode, focuses the diverging energy from a feed antenna into a collimated beam.
Those skilled in the art know that antennas are reciprocal devices, in which the transmitting and receiving characteristics are equivalent. Generally, antenna operation is referred to in terms of either transmission or reception, with the other mode being understood therefrom.
A particular problem with respect to feeds for reflector antennas is that manufacturing costs are relatively high because many parts are required and the overall structure is complex. For example, the structure described in U.S. Pat. No. 4,740,795, Seavey, above is particularly complex and expensive. Often special connectors are required and complex shielding is necessary to prevent leak of electromagnetic radiation. Also, because many different parts are used, each of these has to be tested individually which increases manufacturing time and makes maintenance and repair difficult. These factors increase the cost of feeds which is particularly disadvantageous for domestic systems intended for mass production.
It is accordingly an object of the present invention to provide a dual band antenna which overcomes or at least mitigates one or more of the problems noted above.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.
According to one aspect of the present invention there is provided a dual band antenna comprising:
(i) a single band antenna arranged to operate in a first frequency band and with a first beamwidth; and
(ii) a plurality of single band antenna elements arranged to operate at a second frequency band; and wherein said single band antenna elements are positioned around said single band antenna such that they operate in use with a second beamwidth similar to said first beamwidth.
This has the advantage that a compact, low cost antenna is provided that operates at two frequency bands. In a preferred embodiment said single band antenna is a horn. This gives the advantage that a simple horn to waveguide transition is achieved which simplifies manufacture and thus reduces costs.
According to a second aspect of the present invention there is provided a dual band feed for a reflector antenna said feed comprising:
(i) a single band antenna arranged to operate in a first frequency band and with a first beamwidth; and
(ii) a plurality of single band antenna elements arranged to operate at a second frequency band; and wherein said single band antenna elements are positioned around said single band antenna such that they operate in use with a second beamwidth similar to said first beamwidth.
This has the advantage that a compact and low cost feed is provided that operates at two frequency bands. Also the feed is suitable for use with a reflector antenna in a subscriber outdoor unit, for example, for an interactive television system.
According to another aspect of the present invention there is provided a reflector antenna comprising a dual band feed, said feed comprising:
(i) a single band antenna arranged to operate in a first frequency band and with a first beamwidth; and
(ii) a plurality of single band antenna elements arranged to operate at a second frequency band; and wherein said single band antenna elements are positioned around said single band antenna such that they operate in use with a second beamwidth similar to said first beamwidth.
In this way a low cost, dual band, compact, reflector antenna is formed that can be used for subscriber satellite communication systems such as satellite television.
According to another aspect of the present invention there is provided a method of operating a dual band antenna as described above said method comprising the steps of:
(i) transmitting information input by a user to a satellite using said single band antenna; and
(ii) receiving signals from said satellite using said single band antenna elements, on the basis of said transmitted information.
This provides the advantage that using the dual band antenna a user is able to communicate with a satellite, for example, in a satellite television system. The user is then able to access communications systems to which the satellite is linked, such as the internet.
According to another aspect of the present invention there is provided a method of operating a reflector antenna as described above said method comprising the steps of:
(i) transmitting information input by a user to a satellite using said single band antenna; and
(ii) receiving signals from said satellite using said single band antenna elements, on the basis of said transmitted information.
According to another aspect of the present invention there is provided a one piece carrier casting arranged to support a first single band antenna and a plurality of single band antenna elements and wherein said carrier casting is sized and shaped to support said single band antenna elements at positions around said first antenna. This provides the advantage that a one-piece structure is provided that is inexpensive to manufacture and which is compact. This structure provides support for component parts of a dual band antenna in a cost effective way.
According to another aspect of the present invention there is provided a dual band feed for a reflector antenna comprising:
(i) A single band antenna;
(ii) A plurality of single band antenna elements;
(iii) A one piece carrier casting arranged to support said single band antenna and said single band antenna elements such that said single band antenna elements are positioned around said single band antenna.
This provides a dual band feed that is compact and inexpensive to manufacture. Because a one piece carrier casting is used the positions of the antenna and antenna elements with respect to one another is easily ensured and this reduces manufacturing costs. The one piece carrier is inexpensive to manufacture using known methods.